The IEEE (Institute of Electrical and Electronics Engineers) 902.16 standards propose using Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. OFDMA has also been proposed for use in 3GPP (Third Generation Partnership Project) Evolution communication systems. In an OFDMA communication system, a frequency bandwidth is split into multiple contiguous frequency sub-bands, each sub-band comprising multiple sub-carriers, that are transmitted simultaneously. A user may then be assigned one or more of the frequency sub-bands for an exchange of user information, thereby permitting multiple users to transmit simultaneously on the different sub-carriers. These sub-carriers are orthogonal to each other, and thus intra-cell interference is minimized.
In order to maximize bandwidth usage, for any given Transmission Time Interval (TTI) the sub-bands may be allocated to users based on a reported Channel Quality Information (CQI) value. Further, an appropriate Modulation and Coding Scheme (MCS) is determined for each sub-band and each TTI based on the CQI value. That is, a mobile station (MS) measures channel conditions, such as a Carrier power over the Interference plus Noise power Ratio (CINR), for a common pilot channel or a preamble for each and every sub-band during a measuring period, such as a Transmission Time Interval (TTI) (also known as a sub-frame) or a radio frame transmission period. The MS then reports a Channel Quality Information (CQI) value corresponding to an average of the measured channel conditions across all of the sub-bands to a serving base station or access point in a CQI message. Based on the most recent reported value, a scheduler of an access network serving the MS selectively schedules the sub-bands over a scheduling period, typically one or more TTIs or radio frames, and further determines an appropriate modulation and coding schemes for each sub-band during the scheduling period.
For example, FIG. 1 is a table 100 depicting an exemplary mapping of downlink modulation schemes and coding rates to a CQI value reported in a CQI message. The first column 101 of the table lists ranges of CQI values that may be reported by a MS. Typically, the reported values are integer values. The second column 102 of the table lists the modulation schemes, error encoding rates, and repetition rates, hereinafter collectively referred to as Modulation and Coding Schemes (MCSs), discretely mapped by a scheduler to the corresponding CQI values reported by the MS.
Downlink scheduling based on a reported CQI value provides a simple, straight-forward scheduling scheme. However, in order to best assure acceptable service to all reporting MSs, mapping tables such as table 100 typically have been developed to provide an acceptable MCS to a lowest common denominator among MSs. That is, such tables are based on a ‘one size fits all’ conservative approach to MCS scheduling where, for a given CQI value, the value is mapped to an MCS that is expected to provide acceptable reception for all possible MSs over all possible channel conditions that may be associated with such a CQI value. Such a methodology fails to take advantage of individual variations among the MSs and the environments that they are operating in that may permit more aggressive scheduling for some MSs as opposed to other MSs reporting a same CQI value.
For example, such a methodology fails to consider the sensitivity of an MS's receiver or whether the MS is capable of decoding a particular MCS. Furthermore, the CQI reported by an MS tells little about the fading conditions being experienced by the MS, for example, the CQI fails to distinguish between MSs that report a same CQI value but are experiencing different fading conditions due to travel at different velocities. In addition, use of such mapping tables fail to adequately consider that different MSs, for example, from different MS manufacturers, have different sensitivities and quality of manufacture that may cause them to report different CQI values when operating under identical channel conditions.
Therefore, a need exists for an improved method and apparatus for downlink scheduling in an OFDM communication system.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.